1. Field of the Invention
The present invention relates to an electron gun for use in CRTs (cathode ray tubes) such as a projection tube, a color picture tube and an index tube and, more specifically, to an electron gun in which a plurality of electrode members are coaxially unified with high accuracy.
2. Description of the Prior Art
Conventionally, uni-potential electron guns are widely used for CRTs. FIG. 16 is a schematic sectional view of a uni-potential electron gun. As shown in FIG. 16, the uni-potential electron gun consists of a cathode K for emitting an electron beam, a first grid G.sub.1 and a second grid G.sub.2 that constitute, in combination with the cathode K, a cathode-grid lens, a third grid G.sub.3 that constitutes, in combination with the second grid G.sub.2, a pre-focus lens, and a fourth grid G.sub.4 and a fifth grid G.sub.5 that constitute, in combination with the third grid G.sub.3, a main focus lens. In general, each grid is a cylindrical member made of metal.
In manufacturing the above type of electron gun, the centers of the respective grids need to be positioned on the same axis. To arrange and unify the respective grids, in general, the respective grids are positioned by the outside-diameter reference method and then unified by the glass beading method. This assembling method is described below in detail.
First, as shown in FIG. 17A, a beading jig 100 is prepared on which a plurality of grids having different outside diameters and shapes, for instance, grids G.sub.n and G.sub.n+1 are to be placed. The beading jig 100 has mounting bases A.sub.n and A.sub.n+1 which have been produced by using the outside diameters of a plurality of grids as references so that the grids are rendered coaxial when they are mounted thereon.
Then, as shown in FIG. 17B, the grids G.sub.n and G.sub.n+1 are mounted on the respective mounting bases A.sub.n and A.sub.n+1.
Then, softened bead glasses BG are pressed against fixing parts B.sub.n and B.sub.n+1 of the respective grids G.sub.n and G.sub.n+1 so that tip portions of the fixing parts B.sub.n and B.sub.n+1 are buried in the bead glasses BG. The grids G.sub.n and G.sub.n+1 are fixed to and unified with each other by subsequent cooling (see FIG. 17C).
However, where the grids are coaxially fixed to and unified with each other by the glass beading method with their outside diameters used as references, because of the outside-diameter reference scheme, a maximum axial deviation equal to a sum of outside diameter allowances of two adjacent grids, for instance, will occur between the center axes of those grids. For example, as shown in FIG. 18, where the outside diameter of each of two grids G.sub.n and G.sub.n+1 has a standard 16.+-.0.05 mm (allowance 0.05 mm), an axial deviation S between the grids G.sub.n and G.sub.n+1 amounts to 0.05.times.2=0.1 mm at the maximum if surfaces of mounting bases A.sub.n and A.sub.n+1 of a beading jig 100 are on the same level. This type of axial deviation distorts an electron beam locus and increases lens aberrations. As a result, the size and shape of an electron beam spot on a phosphor screen of a CRT deviate from desired ones, causing a reduction of the resolution.
Further, where grids are fixed to and unified with each other by the glass beading method, since discharging likely occurs between a beading glass and a grid, it is difficult to obtain a high withstand voltage. Although limitations have been imposed on an arrangement of grids and bead glasses and distances between grids have been reduced to prevent the above discharging, the latter attempt has resulted in a new problem that discharging likely occurs between the grids.